In positron emission tomography (“PET”), a radioactive material is placed in the patient. In the process of radioactive decay, this material emits positrons. These positrons travel through the patient until they encounter electrons. When a positron and an electron meet, they annihilate each other. This results in emission of two gamma ray photons that exit the patient traveling in opposite directions. By detecting these pairs of gamma ray photons, one can infer where an annihilation event occurred, and thereby determine the distribution of the radioactive material within the patient.
To detect these pairs of gamma ray photons, (which will now be referred to as “gamma rays”) it is useful to surround the patient with scintillating crystals. When a positron and electron annihilate within the patient, the resulting pair of gamma rays enter opposed scintillating crystals. These gamma rays then interact with the scintillation crystal. In so doing, they cause the emission of an isotropic spray of scintillation photons centered at a point at which the gamma ray interacts with the scintillation crystal. These scintillation photons can be detected by photodetectors in optical communication with the scintillation crystal.
Some of these scintillation photons are emitted in a direction that takes them to the photodetectors. Other scintillation photons, which are emitted in a direction away from any photodetector, nevertheless manage to reach a photodetector after being redirected by structures within the scintillating crystal. Yet other scintillation photons are absorbed and therefore never reach the photodetectors at all.
To detect gamma ray photons, the patient is positioned within a ring of scintillating crystals. Photodetectors observing the crystals can then detect the scintillation photons and provide, to a processor, information on how many scintillation photons were received and from which scintillation crystals they were received. The processor then processes such data arriving from all photodetectors to form an image showing the spatial distribution of radioactive material within the patient.
Each photodetector provides a signal whose intensity indicates the number of scintillation photons reaching that photodetector. Because the photodetector has a large receiving cross section, it is able to detect many scintillation photons. As a result, the photodetector is able to determine, with great precision, when the gamma ray interacted with the material. However, the large receiving cross section of the photodetector limits its ability to provide precise information on where the gamma ray interacted with the scintillating crystal.
To enhance the spatial resolution of a PET scanner, one can place an array of wavelength-shifting, or fluorescent optical fibers in optical communication with the photomultipliers and the scintillation crystal. Scintillation photons can then enter the fluorescent optical fibers. In so doing, the scintillation photons are absorbed. This causes the optical fiber to fluoresce. The photons emitted within the fiber, which will be called “re-emitted photons”, propagate toward a photosensor in optical communication with each fiber. Because the fluorescent optical fibers are much narrower than the photomultiplier tubes, the fiber array provides more spatial resolution than the photomultiplier tubes. This enables the fiber array to provide more precise information on where the gamma ray interacted with the scintillating crystal.
The small diameter of each fiber and the limited probability that the fiber will capture a scintillation photon, means that each fiber collects only a limited number of scintillation photons. As a result, the signal provided by the fiber array provides only limited temporal resolution. This makes it difficult to correlate signals from the fiber array with signals from the photomultipliers, particularly when the intervals between events are short.